This paper compares outcomes of three studies assessing the interplay of formal and informal regulatory systems and market demand (i.e. political economy) on cage-aquaculture practice and environmental performance in Indonesian reservoirs and lakes at a range of scales and market settings. Study sites included two of Indonesia's largest reservoirs in densely populated W. Java, Jatiluhur (83km2, 0.3 km3) and Cirata (62km2, 0.22 km3), and its largest, deepest freshwater lake, Toba, in more remote Sumatra (1,130km2, 240km3).

Exploratory key-informant interviews used to establish context and refine research questions preceded systematic farmer and market surveys. Randomized farmer selection was stratified on 'farm-scale' (using ownership and production indicators) and zonal characteristics (administrative and environmental). Data were collected on cage-infrastructure, seed and feed inputs, harvests over the last full production cycle and production trends. Commercial grower-diets were analysed for total phosphorous (TP) content and results combined with survey-based FCR estimates and satellite-image interpretation for scale/ zone contribution analyses.

In the W. Java reservoirs, 7x7m multi-net 'dua-lapis' cages are used for intensive 3-month culture of mainly <300g carps for local live-sale; outer nets of tilapia and Pangasius utilise waste sinking pellets and on-farm 'wet' feeds for semi-intensive production of secondary crops over 8-24month cycles for more regional cold chain sales and processing. Poorly regulated market-driven expansion by local and (increasingly) external investors has contributed to eutrophication, fish kills and resource conflicts. In Cirata this trend has excluded the smallest farmers, but the 'dua-lapis' system itself has changed little due to its compatibility with local demand, risk management, statutory limits on individual cage size and an implicit policy bias toward local operators. Of an estimated 44,760 cages in Jatiluhur, 35% sited in unapproved zones are entirely locally owned and absent from formal accounts whilst a first-ever attempt to reduce cage numbers in 2014 focused only on unregistered operators in legal zones.

Following failed small-scale development interventions, in Lake Toba, a foreign company initiated large-scale commercial cage-culture in the 1990's to export quality tilapia. Local and regional market demand for larger fish has since been met in part by local cage operators using adapted hybrid-technology in shallow embayments. Small low-cost Javanese style cages are used for intensive monoculture of carp and predominantly tilapia. Co-location with large-scale commercial off-shore operations also gives these farmers access to mono-sex tilapia juveniles and less-polluting extruded floating pellets. This along with the absence of any statutory unit-size limits obviates need for multi-net 'diaper' systems. In the absence of strong formal regulation across study-sites, only the greater natural restriction of small-scale resource-poor farmers to shallower, less exposed littorals in Toba has limited the scale of eutrophication and multi-use resource conflicts experienced in Jatiluhur and Cirata. We suggest policy options for sustainable environmental governance of these aquatic commons acknowledging the primacy of informal and market governance forms.